Peat represents a partially-decomposed organic material formed by an accumulated heterogeneous mixture of partially-decomposed vegetation and inorganic minerals over long periods of time. It forms naturally in wetland conditions in bogs, ferns, pocosins, and peat swamp forests, where flood water obstructs flows of oxygen from the atmosphere, thereby slowing normal rates of decomposition. The resulting pale upper layers contain the remains of the plants, herbs, and moss that died and rotted below the shallow acid water. They are compressed by the weight of the water and other plants to form a fine amorphic, colloidal mass. The lower layers of the peat material constitute approximately 90% water, and resemble mud.
Peat wetlands are found in over 175 countries in the world, and cover around three percent of the world's land area. Significant peat deposits exist in Canada, Finland, the former Soviet Union, Scotland, Ireland, England, France, Germany, and a number of other European countries. The United States also contains significant peat deposits, particularly in Minnesota and Alaska.
Most peat is harvested from these peatland bogs and other wetlands by very large machinery. Generally, the peatland fields are divided into smaller fields by means of perimeter furrows that collect some of the water from the upper layer of the fields. For reed sedge peat, machines are used to dig, chop, and mix the peat from this upper layer and form it into blocks. The excavated peat contains around 95-96% moisture. The peat is then stored in the peatland fields to drain and air dry under the influence of solar heat. Over the course of 9-12 months, the moisture content of the reed sedge peat may be reduced to approximately 80-86%, assuming that little rain falls on the field. Over the course of four years, the reed sedge peat blocks may be reduced to a moisture content of approximately 70%.
Alternatively, the reed sedge peat is disked in the field to fluff up the top layer and then stockpiled in windrows within the until they can be hauled off at a later date. Reed sedge peat is less fibrous than other types of peat, and does not therefore air dry particularly well. Thus, the production costs for field harvesting peat are increased by these long time periods required to reduce moisture content.
Sphagnum peat, on the other hand, is much more-fibrous than reed sedge peat, and is more suitable for air drying on the field. Typically, its top one-inch layer will be disk cut and milled in the field to loosen it up. Then a large vacuum harvester sucks up the loosed Sphagnum peat material and produces around 35% moisture peat. But this is a very expensive, capital intensive process that can only be carried out during a couple months of the year when rain is largely non-existent. Thus, harvesting of Sphagnum peat is a low-volume process.
Peat is used for energy generation, horticultural, and industrial purposes. In many countries, peat is burned as a source of heating fuel, although its moisture content must be further reduced to below 15% to render it an efficient source of thermal heat. Its use in agriculture or horticulture, as a top soil, potting soil, or mulch dressing for retaining moisture in container soil when it is dry, while preventing excess water from killing plant roots during wet conditions, allows for around 60%-moisture material. Peat is also used in freshwater aquaria and water filtration, such as the treatment of septic tank effluent and urban runoff.
American Peat Technology of Aitkin, Minn. has developed proprietary technology for making peat pellets that may be used as carriers for agricultural treatments like Rhizobia microorganisms that produce nitrogen in plant roots, and thermally-activated peat granules that efficiently adsorb heavy metals from wastewater. In conjunction with the University of Minnesota, it has also developed chemically-modified, thermally-activated peat granules that can reduce the leaching of contaminants naturally found in peat into the wastewater during wastewater treatment processes, or selectively adsorb particular heavy metals like cadmium found in the wastewaters at the expense of less-toxic metal cations like zinc. See U.S. Pat. Nos. 8,232,255 and 8,685,884 issued to Green, et al.; and U.S. Ser. Nos. 13/841,526 and 14/213,677 filed by Kolomitsyn, et al., all of which are hereby incorporated by reference. The moisture content of these specialty granulated agricultural and industrial products made from peat must typically be around 1-14%.
Because solar heat and field draining generally are inadequate to reduce the natural moisture in peat down to a level acceptable for these end applications, supplemental drying is required. Manufacturers of peat-related products often employ thermal drying processes to reduce the moisture content of the peat material to an acceptable level. For example, U.S. Pat. No. 7,992,319 issued to Wilson discloses a dryer for coal or peat pellets. Fed into the top of the vertical dryer, the pellets fall through the interior chamber, being exposed to horizontally directed drying gas flow along the way. The dried pellets are removed from the bottom of the dryer chamber.
U.S. Pat. No. 1,290,494 issued to Ten Bosch, N.J. Zoom shows a peat dryer formed by a vertical tower with a decreasing cross-sectional area from the top of the tower to its bottom, ending in a discharge funnel. As the peat is continuously fed through the top of the tower, it is heated by means of a steam supply introduced into the tower chamber. After being subjected to this heat treatment, the peat passes down to a perforated cylinder where the pressure amounts to approximately 79 lbs/in2. Owing to this high pressure condition, water is forced out of the peat to a discharge pipe. The dried peat product falls into the discharge funnel for removal.
U.S. Pat. No. 2,704,895 issued to Cederquist discloses a process in which peat or other vegetable material is fed into a pressure vessel in which it is oxidized under constant elevated temperature and pressure of around 180° C. and 20 atm. Steam and gases are removed from the vessel doing this oxidation process. The oxidized peat is then transferred to an expansion chamber and allowed to expand whereupon steam generated by the pressure drop is discharged from the vessel to further dry the peat. The peat material is then blown to a dryer by means of air. However, this process is limited to peat feed having a dryness of at least 25%. Moreover, these thermal drying processes can be expensive to operate due to substantial equipment costs and the necessary thermal heat for the dryer which can substantially increase operating costs depending upon the combustion fuel used. It is particularly ironic to produce and expend thermal heat for a dryer used to dry peat for use as a combustion product.
Manufacturers in the peat industry have therefore resorted to mechanical processes for dewatering peat. U.S. Pat. No. 5,477,627 issued to Nolin et al. illustrates an attachment mounted to the end of the articulated boom of a carrier vehicle for harvesting peat. The attachment comprises an apertured container having a reciprocating ram. Peat is dug from the bog and placed into the container whereupon the ram extends within the container to reduce the interior volume holding the peat, and force water out through the apertures in the container walls. This attachment can reduce the weight of the material by up to 50%, but considering the comparative density of water versus peat, a significant quantity of the water remains within the peat.
U.S. Pat. No. 4,417,982 issued to Britschgi et al. discloses a wire press machine in which peat is continuously fed along a conveyor screen belt between counter-rotating press rollers to remove water content through holes in the screen. Residual fines are sprayed off the screens, filtered to remove water, and recycled to the peat feed stream. U.S. Pat. No. 4,447,334 also issued to Britschgi et al. dewaters peat by breaking it into 2-3 cm particles, passing the resulting particles along conveyor belts and counter-rotating rollers, and then through a series of nip rollers at increasingly higher pressures to produce peat filter cake.
U.S. Pat. No. 4,526,607 issued to Rosenberg shows a two-step mechanical system for dewatering peat. The first stage comprises of a double machine wire press having a plurality of pairs of rollers that squeeze water out of the peat material as it passes between the counter-rotating rollers. The second stage consists of a filter press featuring at least two successive filter press chambers. The system can reduce the moisture content of the peat feed material from 90% to 45-55%.
But these prior art processes that use roller presses can be problematic. Not only can the equipment, maintenance, and operating costs be substantial, but it has been found that when roller presses are employed to squeeze peat material, the roller press divides the peat into a fines fraction that is lost with the separated water fraction, and a fibrous fraction that is produced by the roller press. This results in a significant change in the composition of the dewatered peat product. For example, peat containing a low concentration of fine particles and extra fibrous particles can be unusable for bacterial growth when the peat is used as a carrier medium for microorganisms.
U.S. Pat. No. 4,543,881 issued to Anderson discloses a dewatering machine for peat or other material having low tensile strength. The machine has an outer roll and an eccentrically-positioned inner roll that squeezes the material to produce severe mechanical compression and shear action. Peat having 90% moisture content can be reduced to 50% moisture content by means of this machine.
U.S. Pat. No. 4,357,758 issued to Lampinen illustrates a system in which peat is placed on a sinter plate saturated with water exposed to water contained in a vessel maintained at sub-atmospheric pressure. A water layer is formed by means of the water on the plate and inside the peat material. The pressure differential across the plate causes water to flow out of the peat through holes in the sinter plate and into the vessel, thereby reducing the moisture content of the peat.
Some manufacturers of peat-based products have resorted to thermo-mechanical dewatering processes. For example, U.S. Pat. No. 4,895,577 issued to Chornet et al. is directed to a system in which the peat is macerated and then mixed with water to produce an 86% moisture slurry. The resulting peat slurry is then heated inside a reactor to a temperature of 160-200° C. by means of steam, followed by mechanical shear and post hydrolysis for a time period of roughly 1-3 minutes. The mechanical shear is preferably achieved by means of passing the slurry through narrow orifices in a nozzle contained in the reactor. The product can then be treated inside a filter press to produce a final peat product having 50-60% moisture. It is quenched rapidly to reduce the temperature to 100° C. or lower in order to terminate the hydrolytic and other reactions within the slurry.
Meanwhile, U.S. Pat. No. 4,525,172 issued to Ericksson discloses a thermo-mechanical system in which peat is dewatered by means of pressing it at a temperature exceeding 90° C. and displacing water contained in the peat by means of warmer water under increasing pressure conditions inside a closed wash press. The peat material is then passed through a mechanical roller press and dried. But the resulting peat material still contains roughly 80% moisture in the press cake before the drying step.
But, these thermo-mechanical systems suffer from a combination of high thermal heat costs and mechanical equipment costs without resulting in a reduction of the peat's moisture to below 50%. Such lower moisture levels are necessary for many combustion, agricultural, and industrial end-use applications for peat. Thus, additional thermal drying will be required to reduce the moisture content of the peat products to acceptable levels despite the usage of the thermal-mechanical dewatering processes.
Some manufacturers have turned to chemical additives to assist with thermal or mechanical dewatering processes. For instance, U.S. Pat. No. 4,720,287 issued to Sheppard et al. teaches a process in which the peat material is heated to 100-150° F. and then treated with a surface active agent like a cationic polymeric surface active agent. The peat is then mechanically pressed at a pressure of 20 atm. Chitosan chloride or an ester of a polyamniocarbonic acid are examples of the chemical agent.
U.S. Pat. No. 8,067,193 issued to Hughes et al. discloses a process for separating solids from a fermentation liquor at a temperature of at least 50° C. using distillation and an anionic polymer selected from natural polymers and modified natural polymers having a high anionic charge such that the equivalent weight is below 300. The anionic monomer units of the polymer are selected from the group consisting of (meth)acrylic acids or salts, itaconic acid or salts, and fumaric acid or salts.
U.S. Pat. No. 6,526,675 issued to Yoon is directed to a method for enhancing fine particle dewatering. A surfactant having a high hydrophile liphophile balance (HLB) member is applied to a slurry of the fine particles. Then a lipid oil coating is applied to the particles. These chemical agents disrupt the bonds between the water molecules and the surface of the material contained in the slurry.
By virtue of the enhanced hydrophobicity, the water molecules are destabilized and more readily removed during the mechanical dewatering process. See also U.S. Pat. No. 7,820,058 issued to Yoon.
Mechanical filter presses have been used in the industry for fiber cell material, although not necessarily for peat. U.S. Pat. No. 6,499,232 issued to Bielfeldt is directed to such a mechanical filter press to which is fed a “sandwich” formed from fine particles placed on a moving belt with coarse particles piled on top of the fire particles. Hot water maintained at 200-220° C. is forced down through the sandwich. The sandwich is then subjected to a downward force inside the mechanical press to squeeze water out of the fibrous material. See also U.S. Pat. No. 6,502,326 issued to Bielfeldt.
Many filter presses use permeable membranes to enhance the separation of water from the feed material during the pressing operation to produce the filter cake with reduced moisture content. See, e.g., U.S. Pat. No. 8,088,288 issued to Whittatker et al. (reverse-phase polymer membrane); U.S. Pat. No. 8,940,173 issued to Bakajin et al. (membrane formed from vertically-aligned carbon nanotubes); U.S. Pat. No. 8,460,554 issued to McGinnis et al. (forward osmosis membrane); U.S. Pat. No. 9,010,544 issued to Miller (microporous membrane); U.S. Pat. No. 6,875,350 issued to Allard (dewatering bag containing permeable membranes and chitosan); and U.S. Published Application 2007/0187328 filed by Gordon (filter sock).
A mechanically demoisturizing process for treating partially-decomposed organic material like peat using a filter press and a permeable membrane that can produce a filter cake with content reduced from about 90% wt or greater to 60% wt without usage of thermal heat or superheated steam would be beneficial. The process would substantially reduce the need for thermal dryers and other expensive mechanical equipment.